U.S. patent application number 17/130068 was filed with the patent office on 2022-06-23 for transfer length phase change material (pcm) based bridge cell.
The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Takashi Ando, Guy M. Cohen, Nanbo Gong.
Application Number | 20220199899 17/130068 |
Document ID | / |
Family ID | 1000005306396 |
Filed Date | 2022-06-23 |
United States Patent
Application |
20220199899 |
Kind Code |
A1 |
Cohen; Guy M. ; et
al. |
June 23, 2022 |
TRANSFER LENGTH PHASE CHANGE MATERIAL (PCM) BASED BRIDGE CELL
Abstract
A tunable nonvolatile resistive element, wherein the device
conductance is modulated by changing the length of a contact
between a phase change material and a resistive liner. By choosing
the contact length to be less than the transfer length a linear
modulation of the conductance is obtained.
Inventors: |
Cohen; Guy M.; (Westchester,
NY) ; Ando; Takashi; (Eastchester, NY) ; Gong;
Nanbo; (White Plains, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Family ID: |
1000005306396 |
Appl. No.: |
17/130068 |
Filed: |
December 22, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 45/144 20130101;
H01L 45/146 20130101; H01L 27/24 20130101; H01L 45/06 20130101 |
International
Class: |
H01L 45/00 20060101
H01L045/00; H01L 27/24 20060101 H01L027/24 |
Claims
1. A neuromorphic device comprising: a phase change material bar,
with the phase change material bar being structured and configured
to have at least two portions that are joined by a first narrow
portion, and with the first narrow portion being located at the
center of the phase change material bar; a resistive liner located
adjacent the phase change material bar, with the resistive liner
being a conduit for conducting at least a portion of a first
electric current; an interfacial layer located between the
resistive liner and the phase change material bar, with the
interfacial layer having a tunable contact resistance; and a set of
ohmic contact portions, with at least one ohmic contact of the set
of ohmic contact portions being located at each end of the phase
change material bar.
2. The neuromorphic device of claim 1 wherein the resistive liner
includes an insulator region located at each end of the resistive
liner, with the insulator region including a material that is
designed to limit the resistive liner span to one transfer
length.
3. The neuromorphic device of claim 1 wherein the contact length is
measured from the end of the amorphous-phase portion of the phase
change material bar to a first end of the insulator region.
4. The neuromorphic device of claim 1 wherein the phase change
material bar includes at least a first crystalline-phase portion
and at least a first amorphous-phase portion.
5. The neuromorphic device of claim 4 wherein at least the first
crystalline-phase portion is located at a first portion of the
phase change material bar.
6. The neuromorphic device of claim 1 wherein the amorphous-phase
portion is initially formed at the first narrow portion of the
phase change material bar.
7. The neuromorphic device of claim 1 wherein the contact length is
adjusted by modulating the amorphous-phase portion of the phase
change material bar.
8. The neuromorphic device of claim 1 wherein the tunable contact
resistance of the interfacial layer is a product of the interfacial
layer composition and thickness.
9. The neuromorphic device of claim 8 wherein the tunable contact
resistance of the interfacial layer is used to tune the contact
resistance between the phase change material bar and the resistive
liner.
10. The neuromorphic device of claim 1 wherein resistance of the
neuromorphic device is modulated by adjusting a first contact
length between the first crystalline-phase portion of the phase
change material bar and the resistive liner.
11. The neuromorphic device of claim 1 wherein resistance drift is
mitigated by conducting the first electric current from the
crystalline-phase portion of the phase change material bar through
the resistive liner during a read operation.
12. The neuromorphic device of claim 10 wherein the resistance of
the neuromorphic device is increased by decreasing the first
contact length between the first crystalline-phase portion of the
phase change material bar and the resistive liner.
13. The neuromorphic device of claim 10 wherein the resistance of
the neuromorphic device is decreased by increasing the first
contact length between the first crystalline-phase portion of the
phase change material bar and the resistive liner.
14. The neuromorphic device of claim 1 wherein the resistance of
the first amorphous-phase portion of the phase change material bar
allows a maximum of one percent (1%) of the first electric current
to flow through the first amorphous-phase portion.
15. The neuromorphic device of claim 1 wherein the electronic
conductance between the set of the ohmic contacts is approximately
linearly proportional to a length of contact between the
crystalline-phase portion and the resistive liner when the length
of contact is less than the length of one transfer length.
16. The neuromorphic device of claim 1, wherein the phase change
material bar is comprised of at least one of
Ge.sub.2Sb.sub.2Te.sub.5, Sb.sub.2Te.sub.3, and GeTe.
17. The neuromorphic device of claim 1, wherein the resistive liner
is comprised of at least one of TaN, amorphous carbon, TiN.
18. The neuromorphic device of claim 1, wherein the interfacial
layer is comprised of at least one of Si.sub.3N.sub.4, HfO.sub.2,
Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 and TaNO.
19. A method comprising: providing a neuromorphic device, with the
neuromorphic device including a crystalline-phase change portion, a
resistive liner, and an engineered interfacial layer; and applying
a RESET pulse to the device to form an amorphous phase change
portion; wherein: the size of the amorphous phase change portion
set the contact length between the crystalline phase change portion
and the resistive liner; and larger RESET pulse yields a shorter
contact length and accordingly lower conductance.
20. A method for assembling a neuromorphic device, the method
comprising: providing a phase change material (PCM) bar, with the
PCM bar including a crystalline-phase portion and an
amorphous-phase portion; placing a resistive liner adjacent to the
PCM bar; placing an engineered interfacial layer between the PCM
bar and the resistive liner so that the engineered interfacial
layer acts as a contact buffer between the PCM bar and the
resistive liner; a set of ohmic contacts on each end of the PCM
bar, with the set of ohmic contacts; and encapsulating the PCM bar.
Description
BACKGROUND
[0001] The present invention relates generally to the field of
phase change materials, and more particularly to an efficient and
practical implementation of phase change materials bridge cell.
[0002] A phase change material or PCM is a material that can be
switched from one phase to another. Based on the properties of the
different phases, PCMs have been explored for their use as a memory
element as well as a tunable resistor for cognitive computing.
Namely, most PCM provides a relatively high resistance when it is
in an amorphous phase, and a relatively low resistance when it is
in a crystalline phase.
[0003] A resistive processing unit (RPU) stores information based
on the resistance of the RPU. During programming, a full SET
operation is used to program the RPU to a low-resistance state
representing a data value such as a logic `1` or a logic `0`. A
subsequent full RESET operation is then used to return the RPU to
its previous high-resistance state. A partial SET or a partial
RESET can be used to tune the RPU to an intermediate resistance
state.
[0004] Conventional PCM RPU devices often employ the PCM material
as a layer disposed between two electrodes. For instance, the PCM
material can be patterned into a bar shape or a fin shape and two
contacts are formed at the opposing ends of the bar. This
configuration is referred to herein as a bridge cell design.
[0005] However, most PCM materials exhibit a resistance drift,
which is typically more pronounced after a RESET pulse is applied.
This drift is undesired for applications where the bridge device is
used as an RPU.
[0006] Accordingly, a RPU designs and techniques for fabrication
thereof with reduced drift would be desirable.
SUMMARY
[0007] According to an aspect of the present invention, there is a
neuromorphic device that includes, but is not necessarily limited
to, the following components: (i) a phase change material bar, with
the phase change material bar being structured and configured to
have at least two portions that are joined by a first narrow
portion, and with the first narrow portion being located at the
center of the phase change material bar; (ii) a resistive liner
located adjacent the phase change material bar, with the resistive
liner being a conduit for conducting at least a portion of a first
electric current; (iii) an interfacial layer located between the
resistive liner and the phase change material bar, with the
interfacial layer having a tunable contact resistance; and (iv) a
set of ohmic contact portions, with at least one ohmic contact of
the set of ohmic contact portions being located at each end of the
phase change material bar.
[0008] According to an aspect of the present invention, there is a
method of operating the neuromorphic device, including the
following operations (and not necessarily in the following order):
(i) providing a neuromorphic device, with the neuromorphic device
including a crystalline-phase change portion, a resistive liner,
and an engineered interfacial layer; and (ii) applying a RESET
pulse to the device to form an amorphous phase change portion.
According to an aspect of the present invention, the size of the
amorphous phase change portion set the contact length between the
crystalline phase change portion and the resistive liner.
Additionally, according to an aspect of the present invention, a
larger RESET pulse yields a shorter contact length and accordingly
lower conductance.
[0009] According to an aspect of the present invention, there is a
method of assembling the neuromorphic device, including the
following operations (and not necessarily in the following order):
(i) providing a phase change material (PCM) bar, with the PCM bar
including a crystalline-phase portion and an amorphous-phase
portion; (ii) placing a resistive liner adjacent to the PCM bar;
(iii) placing an engineered interfacial layer between the PCM bar
and the resistive liner so that the engineered interfacial layer
acts as a contact buffer between the PCM bar and the resistive
liner; (iv) a set of ohmic contacts on each end of the PCM bar,
with the set of ohmic contacts; and (v) encapsulating the PCM
bar.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a block diagram of a first embodiment structure
according to the present invention;
[0011] FIG. 2 is a block diagram of a second embodiment structure
according to the present invention;
[0012] FIG. 3 is a block diagram view of a cross-section of a first
embodiment of a structure according to the present invention;
[0013] FIG. 4 is a block diagram view of a cross-section of a
second embodiment of a structure according to the present
invention;
[0014] FIG. 5 is a block diagram of a top view of a first
embodiment of a structure according to the present invention;
[0015] FIG. 6 is a block diagram of a top view of a second
embodiment of a structure according to the present invention;
and
[0016] FIG. 7 is a graph view showing information that is helpful
in understanding embodiments of the present invention.
DETAILED DESCRIPTION
[0017] Some embodiments of the present invention are directed
towards a tunable nonvolatile resistive element, wherein the device
conductance is modulated by changing the length of a contact
between a phase change material and a resistive liner. By choosing
the contact length to be less than the transfer length a linear
modulation of the conductance is obtained.
[0018] Some embodiments of the present invention provide for a
neuromorphic device. This neuromorphic device includes at least the
following components: (i) a phase change material that is patterned
into a bar that has a narrow portion located at its center; (ii) a
resistive liner; (iii) an interfacial layer that is located
adjacent to the resistive liner and the phase change material; and
(iv) ohmic contacts at each end of the phase change material bar.
In some embodiments of the present invention, the phase change
material bar (sometimes herein referred to as a PCM bar) is
comprised of an amorphous-phase portion and a crystalline-phase
portion.
[0019] In some embodiments, the engineered interfacial layer
composition and thickness is used to tune the contact resistance
between the phase change material and the resistive liner.
Additionally, in some embodiments, the neuromorphic device further
includes an insulator region at each end of the resistive liner,
which effectively limits the resistive liner span on each side to
one transfer length measured from the end of the narrow phase
change bar portion to the beginning of the insulator region.
[0020] Embodiments of the present invention are composed of various
materials. This paragraph will include several non-limiting and
illustrative examples of materials of which various components (as
discussed above) of the neuromorphic device are comprised. The
phase change material bar can be any of the following:
Ge.sub.2Sb.sub.2Te.sub.5, Sb.sub.2Te.sub.3, and/or GeTe. The
resistive liner can include any of TaN, amorphous carbon, and/or
TiN. The interfacial layer can include any of Si.sub.3N.sub.4,
HfO.sub.2, Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 and TaNO.
[0021] In some embodiments of the present invention, the resistance
of the neuromorphic device is modulated by changing the contact
length between the crystalline-phase portion of the PCM bar and the
resistive liner. In some instances, changing the contact length
between the crystalline phase change material and the resistive
liner is done by changing the size of the amorphous phase change
material. To further contextualize the changing of the contact
length, the amorphous phase change material resistance is at least
one hundred (100) times larger than the resistive liner.
[0022] Some embodiments of the present invention may include one,
or more, of the following features, characteristics and/or
advantages: (i) conduction occurs through the resistive liner; (ii)
resistive drift mitigation can take place at any applied level of
resistance; (iii) conductance is approximately linear with the size
of the amorphous region; (iv) contact resistance can be tuned by
choice of liner or by engineering the liner/crystalline-phase PCM
interface; (v) the range of resistance the neuromorphic device can
provide is from the liner resistance (low end) to any high
resistance as long as the current does not flow through the
amorphous region; and (vi) the neuromorphic device can be
implemented as a vertical or horizontal bridge cell. In some
embodiments of the present invention, the effective contact length
(L.sub.C) is modulated by changing the size of the amorphous-phase
portion of the PCM bar when the contact length is less than the
transfer length L.sub.T (that is, L.sub.C<<L.sub.T).
[0023] The transfer length L.sub.T is calculated using the
following formula (F1):
L T = .rho. c R liner + R GST ( F .times. .times. 1 )
##EQU00001##
[0024] In this formula (F1), pc represents the contact resistance
between the liner (such as liner 106, mentioned below in connection
with the discussion of FIG. 1) and the crystalline-phase portion of
the PCM bar (such as crystalline-phase portion 104, mentioned
below). The resistance variable (R.sub.LINER) represents the value
of the sheet resistance of the liner film and the resistance
variable (R.sub.GST) represents the value of the sheet resistance
of the PCM bar material (typically referring to the
crystalline-phase portion). In some embodiments of the present
invention, the liner resistance is chosen to be approximately equal
to the c-PCM resistance, but much lower than the amorphous PCM
(a-PCM) resistance. The liner resistance is chosen such that
R.sub.LINER<<R.sub.a-PCM so that an electric current that is
applied to the PCM bar material flows through the resistive liner
because the R.sub.a-PCM is very high compared to the R.sub.LINER
(as further illustrated in the figures below).
[0025] The following discussion of the figures (FIGS. 1-6) discuss
various embodiments of the present invention, including the
structure, fabrication and methods of operating the PCM bar.
[0026] Diagram 100 of FIG. 1 is a cross section view of a phase
change material bar where the contact length (L.sub.C) is larger
than the transfer length L.sub.T. Diagram 100 includes the
following components: amorphous-phase portion 102,
crystalline-phase portion 104, and resistive liner 106. Since the
contact length (L.sub.C) between the crystalline-phase portion 104
and the resistive liner 106 is considerably larger than the
transfer length (that is, L.sub.C>>L.sub.T), the device
resistance is mostly that of the c-PCM resistance and the resistive
liner resistance. The contact resistance contribution is small.
[0027] Diagram 200 of FIG. 2 is a cross section view of a phase
change material bar where the contact length (L.sub.C) is smaller
than the transfer length L.sub.T. Diagram 200 includes the
following components: amorphous-phase portion 202,
crystalline-phase portion 204, and resistive liner 206.
[0028] Since the contact length (L.sub.C) between the liner and
crystalline-phase portion 204 and the resistive liner 206 is equal
to or less than the transfer length, the device resistance is now
dominated by the contact resistance. The contribution of the c-PCM
resistance and the resistive liner resistance to the total
resistance are small. As the contact length is further reduced the
device resistance will be roughly proportional to 1/L.sub.C, or the
device conductance will be proportional to L.sub.C.
[0029] The applied electric current typically does not flow through
the amorphous-phase portions 102 and 202 because the PCM bar that
is in the amorphous-phase is a poor conductor of electricity.
However, when the contact length is made so short that the
resistance contribution from the contact reaches the resistance of
the a-PCM, the above assumption is no longer valid.
[0030] Diagram 300 of FIG. 3 is a cross section view of the bridge
PCM device following a "weak RESET" which forms an amorphous PCM
region 308 at the narrow portion of the PCM bar. A top view of the
same device is shown in Diagram 500. Diagram 300 includes the
following components: ohmic contact 302, ohmic contact 304,
amorphous-PCM portion 306, oxide layer 308, oxide layer 310,
crystalline-PCM portion 312, engineered tuning interface 314, and
resistive liner 316. As described above in connection with FIG. 1,
the contact length between crystalline-phase portion 312 and
resistive liner 316 is about one transfer length. In some
embodiments of the present invention, the material that is selected
for engineering tuning interface 314 will determine the contact
resistance (pc). The determination of the contact resistance will
ultimately determine the transfer length (as described above in
connection with formula F1). Therefore, the overall resistance of
the PCM bar can be adjusted, or tuned, by changing the material of
engineered tuning interface 314. The total device resistance in
this the case of a weak RESET is approximately the c-PCM portion(s)
resistance plus the liner resistance under the a-PCM.
[0031] Diagram 400 of FIG. 4 is a cross section view of the bridge
PCM device following a "strong RESET" which forms an amorphous PCM
region 406 at the narrow portion of the PCM bar and further extends
into the wider portion of the PCM bar. A top view of the same
device is shown in Diagram 600. Diagram 400 includes the following
components: ohmic contact 402, ohmic contact 404, amorphous-PCM
portion 406, oxide layer 408, oxide layer 410, crystalline-PCM
portion 412, engineered tuning interface 414, and resistive liner
416. The total device resistance in this the case of a strong RESET
is dominated by the contact resistance. The total resistance is
approximately equal to pc/(W*L.sub.C) plus the c-PCM portion(s)
resistance, and the liner resistance under the a-PCM. W is the
width of the contact and L.sub.C is the length of the contact.
[0032] We distinguish between programing operation and read
operation. In a programing operation the amplitude of the RESET
pulse that is applied to the device terminals 402 and 404 is large
and will change the phase of the PCM. The RESET pulse is intended
to melt-quench the PCM so the solidified material is left in the
amorphous phase. The size of the amorphous PCM 406 in the case of a
RESET operation, is proportional to the electric pulses amplitude
that is applied to the PCM bar. Another programing operation is
SET, which leads to a of decrease the amorphous region size or even
have it fully crystalize. The SET pulse is usually smaller in
amplitude than RESET and in most cases the pulse has a longer
trailing edge. Using RESET and SET operations one can tune the
device resistance to a desired value. When a read operation is
performed a small amplitude pulse is used such that the size of the
amorphous PCM region is not changed.
[0033] It is important to note that during a read operation the
conduction of a steady electric current (or electric pulses) that
is applied to the device primarily flows through c-PCM portion 412
to resistive liner 416. No substantial current passes through the
amorphous region 406. This has the effect of reducing the
resistance drift since the majority of the resistance drift occurs
in the amorphous phase of PCM.
[0034] Diagram 500 of FIG. 5 is a top view of the bridge PCM
material during a "weak RESET" operation. Diagram 500 includes the
following components: ohmic contact 502, ohmic contact 504, first
crystalline-phase portion 506, second crystalline-phase portion
508, and amorphous-phase portion 510. In some embodiments of the
present invention, portions 506 and 508 can be thought of as being
the same portion.
[0035] Diagram 600 of FIG. 6 is a top view of the bridge PCM
material during a "strong RESET" operation. Diagram 600 includes
the following components: ohmic contact 602, ohmic contact 604,
first crystalline-phase portion 606, second crystalline-phase
portion 608 and amorphous-phase portion 610. In some embodiments of
the present invention, portions 606 and 608 can be thought of as
being the same portion.
[0036] In some embodiments of the present invention, the PCM bar is
encapsulated (not shown in the Figures). In some cases, the PCM bar
is encapsulated in Silicon Nitride (Si.sub.3N.sub.4), HFO.sub.2,
and many other materials.
[0037] Graph 700 of FIG. 7 is a graph that illustrates the
relationship between the contact resistance (pc) (discussed above
in connection with Formula F1), the transfer length (L.sub.T), and
the resistance of the resistive liner (R.sub.LINER).
[0038] Graph 700 shows that the selection of a R.sub.LINER with a
lower resistance value yields a higher transfer length (or multiple
transfer lengths).
[0039] Some embodiments of the present invention recognize that the
choice of an engineered interface layer material (such as
engineered tuning interface 414) is ultimately determinative in the
strength and polarity of the interface dipole. Some embodiments
additionally recognize the following: (i) the strength and polarity
of the interface dipole can be controlled by geometric mean
electronegativity; (ii) higher geometric mean electronegativity
oxides create higher barrier heights between PCM and liner
material; (iii) for higher pc, high geometric mean
electronegativity dielectric materials should be selected (such as
TiO.sub.2, Al.sub.2O.sub.3, SiO.sub.2); and (iv) for lower pc, low
geometric mean electronegativity dielectric materials should be
selected (such as group 2A and group 3B oxides).
[0040] The following chart (Table 1) is provided as an exemplary
reference for selecting the engineered interface layer materials.
Additionally, the Geometric mean electronegativity values are
calculated using the Sanderson Criterion. The geometric mean
electronegativity values are calculated by using the following
formula: (A.sub.XB.sub.Y).sup.1/x+y.
TABLE-US-00001 TABLE 1 Choice of Engineered Interface Layer Pauling
Dielectric Cap Geometric Mean Element Electronegativity
A.sub.XB.sub.Y Electronegativity Ba 0.89 BaO 1.75 La 1.1 LaN 1.83
Mg 1.31 MgO 2.12 Sr 0.95 La.sub.2O.sub.3 2.18 Hf 1.3 SrTiO.sub.3
2.26 Al 1.61 HfO.sub.2 2.49 Ti 1.54 Al.sub.2O.sub.3 2.54 Si 1.9
AlON 2.56 O 3.44 TiO.sub.2 2.63 N 3.04 SiO.sub.2 2.82
[0041] The descriptions of the various embodiments of the present
invention have been presented for purposes of illustration, but are
not intended to be exhaustive or limited to the embodiments
disclosed. Many modifications and variations will be apparent to
those of ordinary skill in the art without departing from the scope
and spirit of the described embodiments. The terminology used
herein was chosen to best explain the principles of the
embodiments, the practical application or technical improvement
over technologies found in the marketplace, or to enable others of
ordinary skill in the art to understand the embodiments disclosed
herein.
[0042] The following paragraphs set forth some definitions for
certain words or terms for purposes of understanding and/or
interpreting this document.
[0043] Present invention: should not be taken as an absolute
indication that the subject matter described by the term "present
invention" is covered by either the claims as they are filed, or by
the claims that may eventually issue after patent prosecution;
while the term "present invention" is used to help the reader to
get a general feel for which disclosures herein are believed to
potentially be new, this understanding, as indicated by use of the
term "present invention," is tentative and provisional and subject
to change over the course of patent prosecution as relevant
information is developed and as the claims are potentially
amended.
[0044] Embodiment: see definition of "present invention"
above--similar cautions apply to the term "embodiment."
[0045] and/or: inclusive or; for example, A, B "and/or" C means
that at least one of A or B or C is true and applicable.
[0046] Including/include/includes: unless otherwise explicitly
noted, means "including but not necessarily limited to."
[0047] Receive/provide/send/input/output/report: unless otherwise
explicitly specified, these words should not be taken to imply: (i)
any particular degree of directness with respect to the
relationship between their objects and subjects; and/or (ii)
absence of intermediate components, actions and/or things
interposed between their objects and subjects.
[0048] Comprise/comprises/comprising: As used in the specification
(specifically outside of the claims section), this term is intended
to be perfectly synonymous with the term "include" and its various
conjugated forms (as defined herein in this specification). The
term "comprise" (and its various conjugated forms) as used in the
claims is to be interpreted in a manner that is consistent with
patent claim interpretation.
[0049] Without substantial human intervention: a process that
occurs automatically (often by operation of machine logic, such as
software) with little or no human input; some examples that involve
"no substantial human intervention" include: (i) computer is
performing complex processing and a human switches the computer to
an alternative power supply due to an outage of grid power so that
processing continues uninterrupted; (ii) computer is about to
perform resource intensive processing, and human confirms that the
resource-intensive processing should indeed be undertaken (in this
case, the process of confirmation, considered in isolation, is with
substantial human intervention, but the resource intensive
processing does not include any substantial human intervention,
notwithstanding the simple yes-no style confirmation required to be
made by a human); and (iii) using machine logic, a computer has
made a weighty decision (for example, a decision to ground all
airplanes in anticipation of bad weather), but, before implementing
the weighty decision the computer must obtain simple yes-no style
confirmation from a human source.
[0050] Automatically: without any human intervention.
* * * * *